Choosing and Buying The Perfect Greenhouse
Installing a greenhouse in your garden can open up a whole new range of gardening possibilities. Not only will it allow you to protect tender plants over winter, but it will also give you the opportunity to propagate and grow on plants that may later be transferred outside to the garden itself. Greenhouses come in a wide variety of sizes, materials and styles; from timber traditional span designs to aluminium domes. In fact, whatever size or style of garden you have, there’s bound to be a greenhouse design to suit it perfectly.
In this article, we’ll guide you through the process of choosing and building a greenhouse, including selecting the style, size and material, finding the best position, preparing a secure base and assembling the final structure. We also include a handy checklist of features to bear in mind when comparing different styles, sizes and makes of greenhouse.
The majority of gardening that needs to be carried out under glass can take place in a conventional greenhouse. These designs are usually built using a timber or metal framework, with part-solid or all-glass walls.
This style of greenhouse is shaped like a traditional house with an even span roof. The sides may be fully glazed or part-solid. The panes of glass on the roof are usually set so that they overlap slightly to increase rigidity and keep out rain. However, if they overlap too loosely, they may lose heat.
This design is the most practical choice for most gardeners as it offers plenty of growing space and headroom – the best use of space for the least cost.
A Dutch light greenhouse is shaped in the same way as a Dutch barn, with slightly sloping all-glass sides meeting an even span roof. This design, along with its large sheets of glass, lets in the maximum amount of sunlight making it particularly useful for growing low-growing crops such as lettuces. However, as the panes of glass are very big (normally 145×77 cm or 59×30 3/4 in), they can be very expensive to replace.
If your garden does not have enough room for a free-standing greenhouse, you will probably need to consider a lean-to structure. These are usually vertical sided, with a sloping roof that meets the sides in a curve – similar to a conservatory. Alternatively, a shape more like the traditional span may be used, with the span backing on to the wall being half the width of the other side. This is termed a three-quarter span.
Lean-to’s are best sited against beside a sunny house wall; the bricks will retain heat from the sun (and the house heating), which will then be released into the greenhouse. The brick will also provide excellent insulation, which will reduce the amount of escaped heat in comparison to other styles.
Each layer of glazing in a Mansard greenhouse is slightly more slanted than the previous layer, resulting in a greenhouse that is shaped like a tunnel. This allows the maximum amount of sunlight to enter, making this style particularly useful for plants that need plenty of winter light.
Whilst the majority of gardeners will choose a traditional style of greenhouse, there are many others on the market that provide either a more decorative style, specific growing conditions for certain plants or perhaps just good value for money.
Many-sided greenhouses (usually hexagonal or octagonal) are often chosen where appearance, rather than practicality is the major consideration. They tend to be much more expensive than other greenhouses of the same size, and buying equipment and replacement parts is often difficult due to their non-standard shape. There is also less growing space in a polygonal greenhouses in comparison to a traditional one.
Alpine houses are shaped in the same way as a traditional span greenhouse and are designed to keep alpines and other rock plants protected from the damp of winter. The plants require a good flow of air over them, so these houses are usually equipped with louvre vents extending all along the sides. As the alpines do not typically need protection from the cold (only the damp), alpine houses are not heated, making them unsuitable for tender plants.
A dome shaped design is usually the best choice if you need to site a greenhouse in an exposed position, as it offers less wind resistance than traditional greenhouses. Its multi-angled glass panes also provide excellent light transmission. However, the dome shape means that there is a limited amount of headroom around the edge and plants may be difficult to reach.
The mini greenhouse looks somewhat like a glass display cabinet built against a wall. Its small size and low cost makes it ideal for the novice greenhouse gardener. The downside is that its limited space means that only a small number of plants may be grown and all work must be done from the outside. To capture the maximum amount of light, they should be positioned so that they face south-east or south-west.
Planning your Greenhouse Site
If you are planning to install a greenhouse, take some time to really think about where you want to position it so that it blends into your garden design and receives enough light and warmth for the plants to flourish.
Avoid exposed sites as they will make your greenhouse cold – making it expensive to heat or subjecting the plants to the risk of frost damage on cold nights. Likewise, avoid spaces between buildings as they will act as wind tunnels, or areas at the bottom of slopes or next to hedges that can act as frost pockets.
Advance planning is the key to successfully positioning a greenhouse. Make a shade map of the garden throughout the year, marking on the shadows from the house, trees or other large features. You are aiming to find a site that is not in the shade for more than a few hours each day.
If the greenhouse is used mostly in the summer, its longer axis should run north to south to receive the best light. If you are planning to use the greenhouse for over-wintering tender specimens or raising plants in the spring, the sun will be lower in the sky during that time of the year, and so an east-west orientation will provide good light for much of the day. However, it’s worth considering that the dimensions of many small greenhouses (e.g. 6 by 8 ft or 8 by 10 ft) are practically square, so it would be sufficient enough to position any one side in a south or southwest facing aspect for the best exposure to sunlight.
Free-standing greenhouses should be positioned in a sheltered area away from buildings and large trees (trees will cast shade and falling leaves may cover the glass during autumn). A lean-to should be set against a wall that receives both sun and shade – it must receive a certain amount of shade in summer, otherwise it will overheat.
Make sure that the surfaces leading up to the greenhouse are level, preferably with a hard surface and wide enough to accommodate a wheelbarrow. There should be plenty of room in front of the greenhouse door for loading and unloading.
If you want to provide your greenhouse with heat or light, select a site that will be convenient for mains electricity connection; this will also be useful to power timing switches and thermostatic controls. Power can be provided in the garden, but make sure that any outside electrical work is installed by a qualified electrician. You will also need to consider supplying the greenhouse with a tap, particularly if you plan to install an automatic watering system.
Before buying a greenhouse, try to think carefully about the type of work that you intend to carry out there; a greenhouse used mainly for sheltering tender plants will usually have lower space requirements to one used primarily for propagation and growing on. Try not to skimp on the size of the greenhouse you choose, as it will certainly seem much smaller once it is filled with plants. However, do bear in mind that a larger greenhouse will cost more to heat in the winter than a smaller one. One option would be to buy a greenhouse that can be extended; alternatively, thermal screens may be used to partition off unused sections during the winter to save heating costs.
A good minimum practical size for a general-propose greenhouse would be 2 m (6 ft) by 2.5 m (8 ft); smaller structures will be prone to sudden fluctuations in temperature, especially in summer when they may undergo a rapid build-up of heat. A six foot wide greenhouse will allow for a set of 2 ft wide staging on each side, with a 2 foot working area running between them. You will need a wider structure (8 ft) if you need wheelchair access or if you want to be able to bring a wheelbarrow inside.
You may find that many small greenhouses have fairly low eaves and ridges, which may mean that you have to stoop when working at the staging. This can become very tiring even after a short period; consider digging out a sunken base or raising the structure up onto a brick layer to gain more height.
Buying Checklist for Greenhouses
If you are thinking about buying a greenhouse, take the following features into account:
- Check that the ridge height is at least 1.7 m (5 ft 6 in), making sure that you take protruding roof ventilator openings into account.
- The amount of headroom will also depend on the height of the eaves. For a comfortable working space, these should be at least 1.35 m (4ft 6 in).
- Check that the cross bracing on aluminium alloy greenhouses is sturdy enough to make the structure rigid.
- It can be expensive to replace glass of a non-standard size such as those used in a Dutch light greenhouse, so bear this in mind when choosing a design. A standard size is 60 x 45 cm (24 x 18 in) or 60 cm (24 in) square.
- If you are going to need your greenhouse to be consistently warm over the winter, think about installing double-glazing. However, this will increase the cost significantly.
- Bear in mind that the total roof ventilation area should equal one sixth of the floor area – you may need to install additional ventilators.
- Gutters and downpipes are often optional; however, they are very useful for channelling rainwater so that it may be collected and used for watering. They will also prevent rain from flowing from the roof, damaging nearby plants.
- Side ventilators are excellent for providing a through-flow of air during the summer. Make sure that these close tightly to prevent heat loss in the winter.
- Sliding or hinged doors must be at least 60 cm (24 in) wide – more if wheelchair or wheelbarrow access is required, and without a ‘lip’. Sliding doors have the advantage that they can be opened at any width to provide additional ventilation, and will not slam in wind. Make sure that any door shuts tightly and does not let in draughts.
- A kick panel at the base of the door reduces the risk of the glass panel breaking.
- When comparing prices between different greenhouse models or suppliers, bear in mind that a base may be an optional extra. The majority of timber- framed structures will need one.
- ome bases will come with a step, which may make wheelchair access and wheelbarrow use difficult.